The present invention relates to systems and methods for remotely detecting at least one constituent of a gas via infrared detection. A system provided in accordance with the invention comprises at least one source of broadband infrared radiation and a spectrally sensitive receiver positioned remotely from the source. The source preferably features a surface having dimensions at least as large as the receiver's field of view at the distance separating the receiver and the source, and the source and the receiver are oriented such that the surface of the source is in the field of view of the receiver. The source includes a heating component thermally coupled to the surface, and the heating component is configured to heat the surface substantially uniformly to a temperature above ambient temperature. The receiver is operable to collect spectral infrared absorption data representative of a gas present between the source and the receiver. The system of the invention advantageously overcomes significant difficulties associated with active infrared detection techniques known in the art, and provides an infrared detection technique with a much greater sensitivity than passive infrared detection techniques known in the art.
As a background to the invention, much renewed emphasis has been placed in recent years on measuring trace gas constituents of the atmosphere, and researchers continually attempt to develop more sensitive methods to assess the state of the environment. Most countries now regulate or restrict atmospheric pollution and many have dedicated agencies (such as the EPA in the United States) to monitor gas-phase pollution levels. Hence, great emphasis is being given to developing reliable, near real-time sensor systems to monitor changes of industrial effluents. A variety of techniques have been described in the literature for using a Fourier Transform Infrared (FTIR) spectrometer for remote detection of a gas. Remote sensing with FTIRs roughly falls into two rather broad categories, namely, systems that are “active” in nature versus those that are “passive” in nature.
In an active system, the distinguishing characteristic is that there is an active source of infrared (IR) radiation, i.e., an artificial bright source of IR light, which is focused and collimated to provide a high-energy TR beam. Examples of active sources described in the literature include, for example, an SiC glow bar or a Nichrome wire element powered by a low-voltage high current DC source. The radiation is collimated using a telescope, and the beam is projected across the desired area and focused onto an IR detection system. In an alternative active method, called a monostatic setup, the source and detecting spectrometer stand together. The propagated IR beam is focused upon a remote retroreflector in a manner whereby the beam is reflected back to the receiver. The monostatic case is complicated by the fact that the returned beam must be precisely received by the receiver port in order to be focused onto the detector. In either case, active systems confer the advantage that the infrared detector sees a very large temperature difference between the background and the target gas plume (i.e., there is a hot active IR source). Typical sources may operate near 1350 Kelvin (K), and for measurements made near the earth's surface, ambient temperatures are near 275±40 K, thus providing a temperature contrast between sample and background of about 1000 K.
Although active FTIR spectroscopy offers excellent sensitivity, the logistics of trying to align the sender and receiver telescopes poses a serious problem. Successful employment of the active mode requires careful co-alignment of the IR sender and receiver telescopes, typically to better than 5 arc-sec. For a typical telescope clear aperture of about 25 cm (i.e., about 10 inches) and typical separations of 50 to 500 meters, it is extremely difficult to align the optical paths of the sender and receiver telescopes using commonly available mechanical means. Moreover, detection is only accomplished over the path defined by the set-up, and is not easy to readjust by angular sweeps because the sender and receiver telescope bores must not only be parallel, but must be co-axial as well, which can be difficult to achieve and maintain, usually requiring radio communication. Monitoring even a slightly different path requires a new alignment, which is cumbersome, requires remote communications, and is typically very time consuming, often requiring several hours. The analogy has been made that it is similar to “two rifles trying to shoot each other down the barrel,” e.g. requiring alignment to better than 1 arc-min and approaching 2 to 5 arc-sec.
Maintaining both position and orientation is crucial in an active FTIR remote sensing system because an FTIR detector registers only the time-dependent modulation of intensity. As a consequence, even small arbitrary intensity fluctuations while recording the interferogram result in bogus features in the spectrum obtained from the FT algorithm. The active monostatic configuration combines the sender and receiver as a single bore-sighted unit with a retroreflector used to return the IR beam to the sender, alleviating many of the field alignment difficulties; however, sighting to the retroreflector within its FOV can still be formidable and the method presents other challenges for good signal recovery. Since the IR light does not emanate from a point source, the divergence of the beam can be a limiting factor since twice the nominal distance is covered, and the divergences of the outgoing and return beams seriously limit the radiation-gathering ability. At large separations the retro aperture(s) must be large to reduce their contribution to overall beam divergence. Finally, the outgoing and return beams also need to be separated; the simplest duplexing method is by a beam splitter, meaning that at best 25% of the light gathered from the source is seen by the detector. The divergence and reflective losses typically mean that less than about 10% of the gathered light is seen by the detector.
The alternative to the active IR detection method is passive FTIR spectroscopy for standoff detection of chemical signatures. The main advantage of the passive technique is that it has only a receiver optic and spectral sensor. There is no active source, which eases many logistical and operational requirements. The technique uses the ambient thermal radiation of the earth as the source of infrared light. If a gas cloud is at a temperature that is colder than the background temperature, an absorption signature is seen (provided the gas has an IR absorption spectrum, as most do). Should the gas temperature be greater than the background temperature, the chemical signature will be seen in emission mode. The passive configuration is sensitive for near real-time analysis because the detector (typically, a liquid N2-cooled semiconductor at 77 K) is colder than the surroundings and is thus capable of detecting the incoming IR radiation. That is to say, since the detector is at about 80 K, and the earth is typically near 300 K, there may exist a difference in the radiometric output of the cloud and its background, and the detector is cold enough to register this difference. When the “on-plume” signature is seen in emission mode, the cloud is at a temperature hotter than the background temperature, and the chemical signature peaks are upward going, typically atop a broad spectral background due to the instrumental response. To remove the background and improve the sensitivity of the technique, an “off-plume” spectrum is normally recorded, and this is subtracted from the “on-plume” spectrum. This can be done simultaneously at the level of the interferogram providing more sensitive differencing. Ideally, the difference spectrum has a simple flat background with only the signature peaks visible. Should the background temperature be greater than the plume temperature, the same signatures are seen in absorption mode. Clearly, the temperature difference plays a crucial role in the nature of the signal and its sensitive recovery.
In either absorption or emission, however, the greatest limitation of the passive technique is usually the lack of significant thermal contrast between the plume and its background. Even when plume and background are at the same temperature it is theoretically possible to still measure the plume if its emissivity is significantly different from that of the background, because there is still a difference in radiance; however, in practice, the difference in emissivity of the background and foreground are similar in scale, so spectral brightness differences are hard to exploit. For passive FTIR there are some circumstances in which one can anticipate a large ΔT for passive sensing. For example, a stack emission could be known to be at a very high temperature, such as about 200° C., in relation to an ambient background. In many cases, however, this cannot be guaranteed, or stack-release temperatures can be highly variable, or possibly even intentionally changed. In such cases, it would be very advantageous to be able to guarantee a minimal thermal contrast that is as large as possible.
Most FTIR work exploits a narrow portion of the thermal IR spectral region. For example, passive remote sensing techniques are generally limited to the long-wave infrared (LWIR) atmospheric window in the 1300 to 700 cm−1 (7 to 14 μm) range. The CO2 band at 667 cm−1 and many water rotational lines generally obscure the region below 700 cm−1, while the water bending band centered at 1604 cm−1 obscures much of the region from 1300 to 1900 cm−1, thus leaving only a 700 to 1300 cm−1 window for passive studies. Spectral information in the mid-IR is also useful, especially for monitoring CO and acid halides, but has very low ambient flux making passive detection of the molecules much less sensitive. Although passive FTIRs can monitor a wide diversity of compounds in the thermal IR (LWIR) atmospheric window, and therefore are a natural choice for air quality testing, they are not sensitive to acid halides and some light IR active gases such as NO or CO which are emitted by many exhaust stacks.
In view of the above background, it is apparent that there is a continuing need for further developments in the field of remote gas sensing using infrared detection. In particular, there is a need for further advancement in the development of techniques that are easy to use and provide a sensitivity that is capable of providing useful information to an operator. The present invention addresses these needs, and further provides related advantages.